1. Field of the Invention
The present invention relates to a broadband lossless dipole antenna. The invention has been developed based on the requirements that exist in connection with broadband low-frequency radar, especially a type called CARABAS-II which uses the frequency range 20-90 MHz and requires an antenna with a relatively great beam width. In the following discussion, reference is made to this concrete example. How-ever, the antenna can also be used in other applications where a broadband antenna with a great beam width is desired and in other applications as well, and it is Applicant's pronounced opinion that the antenna should merit protection merely based on its construction and independently of its application.
2. Description of the Related Art
If a common dipole antenna should be used with the broadband low-frequency radar CARABAS-II, there will be problems with beam splitting at higher frequencies. FIG. 1 illustrates an example of a conventional dipole antenna with a central feeding point M, and FIG. 2 is an antenna gain chart for such a dipole antenna. The main beam, the central area with a high antenna gain, should essentially be constant over the frequency range. In the right part of the chart it is, however, to be seen that the problem with beam splitting starts to appear at about 75 MHz. A common dipole antenna thus is not suitable in the current case.
Up to now, CARABAS-II has used, instead of a common dipole, a dipole antenna in the form of a loaded slim biconical antenna, 4.9 m long. However, this antenna has an average efficiency over the frequency band of about 55% only, a higher efficiency being desirable. The reason for the low efficiency is that the antenna is provided with two filters, one on each dipole arm. The filters consist of parallel inductances, capacitances and resistances and are placed a distance onto the dipole arms. The filters prevent currents on the outer part of the dipole arms at higher frequencies, which prevents beam splitting occurring at the higher frequencies. A great part of the power supplied to the antenna is, however, absorbed in the resistances of the filters.